Semiconductor devices are used for integrated circuits in a variety of electrical and electronic applications, such as computers, cellular telephones, radios, and televisions. One particular type of semiconductor device is a semiconductor storage device, such as a random access memory (RAM) device. Many RAM devices include many storage cells arranged in a two-dimensional array with two sets of select lines, wordlines and bitlines. An individual storage cell is selected by activating its wordline and its bitline. RAM devices are considered “random access” because any memory cell in an array can be accessed directly if the row and column that intersect at that cell are known.
A commonly used form of RAM is known as a dynamic RAM (DRAM) device. Dynamic random access memory has memory cells with a paired transistor and capacitor, wherein the capacitor requires periodic refreshing. One particular type of DRAM device is a synchronous DRAM (SDRAM) device, in which the memory cells are accessed synchronously. Synchronous dynamic random access memory often takes advantage of the burst mode concept to greatly improve performance by staying on the row containing the requested bit and moving rapidly through the columns. To achieve a high speed operation, a double data rate (DDR) architecture is often used, during which two data transfers are made per clock cycle, one upon the rising edge of the clock and the other upon the falling edge.
Normally multiple DRAMs are connected in parallel to the data lines to achieve high memory capacity with low cost. One issue that arises with high speed operation is the need to properly terminate bus lines. Each chip along a bus line will cause an impedance discontinuity, which can cause reflections and other noise such as distortion and ringing. To minimize these affects, the bus will be terminated with a resistor, with the goal of having uniform impedance along the line. A technique known as on-die termination (ODT) can be used to terminate the bus on the chip itself rather than on the circuit board.
Although the termination resistors on the circuit board reduce some reflections on the signal lines, they are unable to prevent reflections resulting from the stub lines that connect to the chips. A signal propagating from the controller to the components encounters an impedance discontinuity at each stub leading to the chips on the module. As a result, a reflection will be generated at each stub independent of the termination setting. It is desired, however, to minimize the impact of these reflections.